I bought these thinking they would fit as long as I had hub rings:

TSW Tanaka 20 x 9 0 10 5 F Black Rims Wheels BMW x6 Xdrive 35 | eBay

I used an offset calculator before I bought, but the tire place showed me the fit and said spacers for the rear are needed. I've never used spacers so what am I looking for? Links would be great! Thanks!